Built-in Modules
    Built-in modules are compiled directly into PHP and carried around
    with every PHP process; their functionality is instantly available
    to every script that's being run. Like external modules, built-in
    modules have advantages and disadvantages, as described in the
    following table:
    
 Built-in modules are best when you have a solid
    library of functions that remains relatively unchanged, requires
    better than poor-to-average performance, or is used frequently by
    many scripts on your site. The need to recompile PHP is quickly
    compensated by the benefit in speed and ease of use. However,
    built-in modules are not ideal when rapid development of small
    additions is required.